Two classes of escherichia coli mutants have been isolated with altered permeability. Genetic analyses indicated that the genetic alterations involved are distinct in these two mutants. One mutant, designated NbsB, carries a mutation which effects the synthesis of the aldoheptose, L-glycero-D-mannoheptose. It is postulated that L-glycero-D-mannoheptose is formed from D-glycero-D-mannoheptose via an epimeration step. A temperature-sensitive mutant has been isolated with the NbsB phenotype. A crude assay for the epimerase activity and selected hybrid ColEl plasmids capable of correcting the NbsB phenotype and increasing the epimerase activity 6- to 10-fold relative to the wild type strains, has been developed. This represents the first direct biochemical evidence of aldoheptose biosynthesis.